<!DOCTYPE html>
<link rel="help" href="https://drafts.csswg.org/css-scroll-snap-1/#valdef-scroll-snap-type-mandatory" />
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="./support/common.js"></script>
<style>
div {
  position: absolute;
  margin: 0px;
}
#scroller {
  height: 500px;
  width: 500px;
  overflow: hidden;
}
.child {
  width: 300px;
  height: 300px;
  background-color: blue;
}
</style>

<div id="scroller">
  <div class="child" style="top: 0px; left: 0px;"></div>
  <div class="child" style="top: 1000px; left: 1000px;"></div>
  <div style="width: 2000px; height: 2000px;"></div>
</div>

<script>
test(() => {
  scroller.style.scrollSnapType = "both mandatory";

  // Scroll to where the first child is in view.
  scroller.scrollTo(100, 100);
  assert_equals(scroller.scrollLeft, 100);
  assert_equals(scroller.scrollTop, 100);

  // Scroll to where the second child is in view.
  scroller.scrollTo(900, 900);
  assert_equals(scroller.scrollLeft, 900);
  assert_equals(scroller.scrollTop, 900);
}, "No snapping occurs if there is no valid snap position");

test(() => {
  scroller.style.scrollSnapType = "x mandatory";

  for (const target of document.querySelectorAll(".child")) {
    target.scrollSnapAlign = "start none";
  }

  // Scroll to where the first child is in view.
  scroller.scrollTo(100, 100);
  assert_equals(scroller.scrollLeft, 100);
  assert_equals(scroller.scrollTop, 100);

  // Scroll to where the second child is in view.
  scroller.scrollTo(900, 900);
  assert_equals(scroller.scrollLeft, 900);
  assert_equals(scroller.scrollTop, 900);
}, "No snapping occurs if there is no valid snap position matches scroll-snap-type");

promise_test(async t => {
  // Start with valid snap positions.
  scroller.style.scrollSnapType = "y mandatory";
  document.querySelectorAll('.child').forEach(el => {
    el.style.scrollSnapAlign = 'start';
    t.add_cleanup(() => {
      el.style.scrollSnapAlign = '';
    });
  });
  scroller.scrollTo(100, 100);
  await waitForNextFrame();
  const scrollPosition = scroller.scrollTop;
  // Elements no longer snap along the y-axis.
  document.querySelectorAll('.child').forEach(el => {
    el.style.scrollSnapAlign = 'none start';
    // Bump the position to verify that we don't stay pinned to the same element
    // after layout update.
    el.style.top = `${parseInt(el.style.top) + 100}px`;
  });
  await waitForNextFrame();
  assert_equals(scroller.scrollTop, scrollPosition);
  scroller.scrollTo(900, 900);
  assert_equals(scroller.scrollLeft, 900);
  assert_equals(scroller.scrollTop, 900);

}, "No snapping occurs when last remaining valid snap point is no longer " +
   "valid.");
</script>
